1. Field of the Invention
The present invention relates to a video camera apparatus provided with an automatic focus adjusting device for automatically bringing a subject into focus.
2. Description of the Related Art
It is known that an apparatus, such as a video camera, having a two-dimensional image pickup device utilizes a method of detecting the sharpness of a picture from a video signal of a subject, controlling the position of a focusing lens so that the sharpness becomes a maximum, and bringing the apparatus into focus.
In general, to evaluate the sharpness signal, either of the following signal strengths is employed: the strength of a high-frequency component of a video signal extracted by a band-pass filter or the detection strength of a defocusing width (the width of the edge portion of a subject image) contained in a video signal extracted by a differentiating circuit or the like. In a case where an ordinary subject is photographed, if the focusing lens is out of focus, for example, the level of the high-frequency component of the video signal is small, but as the focusing lens approaches an in-focus point, the level of the high-frequency component becomes larger. If the focusing lens completely reaches the in-focus point, the level of the high-frequency component reaches a maximum. Accordingly, if the aforesaid sharpness signal is small, the focusing lens is driven at as high a speed as possible in the direction in which the sharpness signal becomes greater, and as the sharpness signal becomes greater, the driving speed of the focusing lens is made lower so that the focusing lens can be made to precisely stop on "the top of a hill", i.e., so that the focusing lens can be brought into focus. Such an autofocus method is generally called a hill climbing autofocus system (hereinafter referred to as "hill climbing AF").
Conventionally, an area from which to extract the sharpness signal required for the hill climbing AF (hereinafter referred to as "distance measuring frame") is provided in the approximately central portion of a picture (refer to the frame A shown in FIG. 3) on the assumption that many photographers usually position a main subject at the central portion of a picture.
However, if the distance measuring frame is fixed in the central portion of the picture, the following problem will be experienced. If a subject which is being photographed moves out of the distance measuring frame, it is determined that the subject has disappeared, and the focusing lens is made to move in a direction in which defocusing occurs, from its current focus position at which the focusing lens is in focus, with the result that unnecessary defocusing occurs.
To solve the above-described disadvantage, a control system is considered. In the control system, the size of a distance measuring frame is made variable so that it is possible to always select an optimum distance measuring frame by cyclically switching the size of the distance measuring frame between different sizes.
According to the control system, during restarting of driving of the focusing lens which is in focus, it is possible to perform an AF operation using an optimum distance measuring frame, whereby it is possible to effect a rapid and reliable AF operation.
However, in the above-described example, since a distance measuring frame to be used during zooming is fixed to a small distance measuring frame, if zooming is performed, for example, in the vicinity of a telephoto end during AF control using a large distance measuring frame, the large distance measuring frame is switched to the small distance measuring frame and, particularly in the case of zooming of from a wide-angle side to a telephoto side, a subject which has so far been recognized may be lost sight of. At this time, the photographic image within the picture is greatly defocused.
Regarding the control of such a distance measuring frame, there is another problem to be considered. As is known, cameras having a so-called electronic-zoom function for electronically enlarging a photographic image have recently become popular. Such a camera can realize higher-magnification zooming by combining the electronic-zoom function and an optical-zoom function, whereby it is possible to cope with various photographic conditions.
There are two methods for setting a distance measuring frame from which to obtain an AF sharpness signal relative to an angle of view which is varied by the electronic zoom. One method is to extract the AF sharpness signal from a video signal contained in the picture of an image enlarged at an image magnification, and another method is to extract the AF sharpness signal from a video signal before enlargement.
In the former case in which the AF sharpness signal is extracted from a video signal contained in the picture of an enlarged image, a high-frequency component is actually extracted from a video signal contained in the picture of a picked-up image which is horizontally thinned out, and the thus-obtained high-frequency component differs from a high-frequency component which can be extracted from the original video signal. This leads to the problem that the accuracy of an AF operation degrades or an erroneous operation easily occurs. To solve such a problem, it has been proposed to provide means for extracting an AF sharpness signal from a video signal before enlargement as preferentially as possible.
However, during an electronic-zoom operation, in a case where a video signal before enlargement is used for AF control to prevent degradation of the accuracy of an AF operation, if the size of a distance measuring frame is greater than the size of an area to be enlarged, a subject which is located outside of the picture of an enlarged area may be brought into focus.
Also, during AF control using a large distance measuring frame, if the size of the distance measuring frame is made smaller just because the electronic zoom is activated, a subject which has been recognized up to that time may be lost sight of. At this time, the focusing lens moves to a great extent, with the result that defocusing occurs or it takes a long time to again bring the focusing lens into focus.